This invention relates to a retorting process for recovering product hydrocarbons from oil shale and other hydrocarbon-bearing solids. The invention most particularly relates to those oil shale retorting processes wherein heat energy in retorted shale particles is used to recover product hydrocarbons from raw shale fines.
Many methods for recovering oil from oil shale have been proposed, nearly all of which utilize some method of pyrolytic eduction commonly known as retorting. To be competitive with the production of oils from petroleum stocks, one difficulty to be overcome is the recovery of essentially all heat value from carbonaceous material in the shale without incurring prohibitive expense or environmental damage. Since shale usually contains only about 20 to 80 gallons of oil per ton, only a limited proportion of which can be recovered as product oil or gas, economical retorting must utilize remaining heat energy contained in the shale to provide heat for pyrolytic eduction. However, sulfur emissions in flue gases released from the retorting process must be restricted to the low levels required by law while this goal is being attained.
It is known to retort oil shale by a technique of contacting up-flowing oil-bearing solids with downflowing gases in a vertical retort, and one such technique is disclosed in U.S. Pat. No. 3,361,644. To educe product vapors, the upward-moving bed of shale particles exchanges heat with a downflowing, hydrocarbonaceous and oxygen-free eduction gas of high specific heat introduced into the top of the retort at about 950.degree. to 1200.degree. F. In the upper portion of the retort, the hot eduction gas educes hydrogen and hydrocarbonaceous vapors from the shale and, in the lower portion, preheats the ascending bed of particles to retorting temperatures. As preheating continues, the eduction gas steadily drops in temperature, condensing high boiling hydrocarbonaceous vapors into a raw shale oil product while leaving a product gas of relatively high BTU content. The shale oil and product gas are then separated, and a portion of the product gas, after being heated, is recycled to the top of the retort as the eduction gas.
To minimize the volume of the recycle gas required, upflow retorting is usually conducted under superatmospheric pressure with the pressure in the upper regions of the retort often being between 10 and 50 p.s.i.g. However, means must be provided for introducing and recovering granular shale from the superatmospheric retorting zone without allowing valuable product and recycle gases to depressure. Conventional methods for achieving these objectives use elaborate lock vessels, valves, star feeders, or slide valves, which tend to wear rapidly and produce excessive fines through abrading the shale. Alternatively, liquid sealing devices, as in U.S. Pat. No. 4,004,982, have been employed, which operate by moving shale particles through a standing head of oil or water, thereby creating a positive back pressure to forestall escape of retort gases. Liquid seals effectively contain retort gases but leave the shale wet. When incorporated into a process for combusting retorted shale in a vessel separate from the retort, use of liquid seals would require the expense of drying the shale prior to combustion.
To increase product yield beyond what can be educed in the retort alone, processes have been developed to generate product gases by reaction of hot, retorted shale with an oxidizing gas stream, for example, as taught in U.S. Pat. No. 4,010,092. However, such gasification reactions conducted in an oxidizing environment burn the coke on retorted shale at temperatures high enough to release significant amounts of carbon dioxide from decomposing carbonates in the shale, thereby necessitating expensive removal of carbon dioxide from combustible product gases.
Another source of product yield is unretorted shale fines. Shale mined for the purpose of retorting in above-ground retorts is usually crushed mechanically to a size suitable for retorting, for example, about three inches in diameter, or smaller. Due to the friable nature of shale, fines ranging in size up to about one-half inch in diameter are generated in the mining and crushing of larger retort-sized particles in amounts up to about 10 weight percent of the total shale mined. In processes developed for use with above-ground retorts, fines mixed into the feed of larger, retort-sized particles tend to fill the void spaces between the larger particles and agglomerate during retorting. As a result, circulation of hot eduction gases is channeled into the few available unfilled passageways through the voids, which consequently overheat, while circulation of heat to the rest of the retort is blocked off, leaving larger underheated areas. When fines are segregated from the feed to the retort to avoid this problem, an appreciable portion of the energy available from the shale is wasted.
Retorted shale contains heat value in the form of coke, and many retorting processes pass retorted shale particulates through a combustion zone to combust the coke and thus recover heat energy. However, because retorted shale generally contains sulfur components, less than complete combustion of the coke generates hydrogen sulfide, which must be removed from flue gases by means of costly sulfur recovery processes. On the other hand, complete combustion may result in flue gases containing unacceptable amounts of sulfur dioxide. To solve the problem of sulfur dioxide production during complete combustion, U.S. Pat. No. 4,069,132 discloses a combustion process wherein the sulfur dioxide generated during the combustion of coke on the retorted shale is converted to stable inorganic salts by reaction with alkaline ingredients of the shale. This process utilizes a combustor through which hot, retorted shale gravitates cocurrently with air for combustion diluted by sufficient flue gas to control peak combustion temperature below 1670.degree. F. Under such conditions, the discharge of sulfur dioxide from the combustor is disclosed to be greatly minimized.
Because flue gases from combustion zones associated with shale retorts are usually at high temperature, many retorting processes recover heat therefrom. For example, as taught in U.S. Pat. No. 4,069,132, the hot flue gases may be utilized to exchange heat indirectly with boiler feedwater to generate process steam.
While the aforementioned features have met with some success, the need exists for further developments in shale retorting processes. For example, the need exists for a process by which raw shale fines can be retorted to educe and recover hydrocarbon products using heat contained in shale particles removed from an oil shale retort and without necessitating separate facilities for recovering and condensing the hydrocarbon products so educed from the fines.
Accordingly, a principal object of this invention is to provide a process for recovering up to 100 percent of the Fischer assay of hydrocarbons from raw shale fines utilizing the heat contained in retorted shale particles removed from a shale retort to educe product vapors from the fines while additional product vapors are educed from the retorted shale particles.
Another object of this invention is to provide a process for removing retorted shale particles from a retort while sealing the retort gases therein, crushing the particles under elevated pressure to a size suitable for fluidization, and subsequently holding the crushed particles as a fluidized bed to aid in sealing the retort while using heat energy contained in the crushed particles and in recycle retort gases to effect flash pyrolysis of unretorted fines so that up to 100 percent of the Fischer assay of product hydrocarbons contained therein is recovered.
Yet another object of the invention is to retort raw shale fines using a particulate heat source held as a fluidized bed to facilitate heat exchange and prevent agglomeration of the fines.
It is another object of the invention to integrate the foregoing process and apparatus into an overall process for retorting oil shale, recovering the retorted shale without loss of retort gases, using the retorted shale to educe product hydrocarbons from unretorted shale fines under fluidized conditions while achieving secondary retorting of the retorting shale and, optionally, combusting residual coke on the retorted shale under fluidized conditions for the purpose of recovering heat derived from the combustion of said coke.
These and other objects of the invention will become more readily understood in light of the following description and claims.